The present invention relates to a process for the preparation of amino acids, particularly by hydrolysis of acylamino acids.
Amino acids are used in human medicine and in the pharmaceuticals industry, in the foodstuffs industry and, very particularly, in the feeding of animals. Amino acids can be in enantiomerically pure form, but also in the form of the racemate. Amino acids are also used in the synthesis of numerous fine chemicals and active ingredients.
Various processes are available for the industrial production of amino acids, such as the hydrolysis of proteins, the Strecker process, biotechnological processes and amido-carbonylation.
The reaction known as amidocarbonylation is the conversion of carbonyl compounds or alkenes with acylamides as the nitrogen source and carbon dioxide or synthesis gas. The first products of that reaction are not the free amino acids, but the N-acylamino acids. Such N-acylamino acids are valuable starting materials for the enzymatic racemate cleavage for the preparation of enantiomerically pure amino acids (Beller et al., Chem. Eur. J. 1998. 4, 935-941).
It is, however, not important to use an enantiomerically pure form in all applications of amino acids. There is also a need for racemic amino acids. An example thereof is D,L-methionine, a racemic amino acid, which is used in large amounts as a feed additive for animals and poultry.
The hydrolysis of acylamino acids to amino acids is usually carried out by reaction with aqueous lye or mineral acid in at least equimolar amounts. Examples thereof are given, for example, in an article by J. F. Knifton (Catalysis Today, 18 (1993) 355-384). In order to isolate the amino acid, the lye or mineral acid that has been added must then be neutralized once the reaction is complete. Owing to the in some cases good solubility of amino acids and the salts in water formed on neutralization, the separation from the salt on isolation of the pure amino acids is often associated with considerable expense and losses. When ion exchangers are used, salt formation is not avoided but simply moved to a different process stage. The large amount of salt that is inevitably formed in that process is highly disadvantageous for an industrial application from an ecological and an economic point of view.
It is also known that amides can be hydrolyzed with water in the region of the critical point (374xc2x0 C., 218 atm) without the addition of acids or lyes. Under such conditions, however, the technical requirements are considerable. The reaction medium is highly corrosive in that temperature range and high-quality materials are therefore required for the construction of reaction equipment.
In consideration of the prior art outlined and discussed above, it is an object of the present invention to find a further process for the preparation of racemic amino acids from acylamino acids, in which the amino acids can readily be isolated and the formation of large amounts of salts is avoided.
The above and other objects of the present invention can be achieved by a process for the preparation of racemic amino acids, wherein an acylamino acid of the general formula
R1xe2x80x94CH(NHxe2x80x94COxe2x80x94R2)COOH
wherein R1 is hydrogen, linear, branched or cyclic alkyl that has from 1 to 7 carbon atoms and that may contain substituents such as a hydroxyl, alkyloxy or alkylthio group; and R2 is a hydrogen atom or an alkyl having from 1 to 3 carbon atoms;
is heated in the presence of water, in a pressure-resistant vessel, to a temperature in the range from 110xc2x0 C. to 220xc2x0 C. and is hydrolyzed.
It has been found that acylamino acids can also be hydrolyzed in a temperature range that is markedly below the critical temperature without the addition of acids or bases.
An important aspect according to the present invention is that the required temperature range that is necessary in the process according to the invention is markedly below the critical temperature, which means that the demands made of the materials and the required pressure resistance of the reaction vessel lie within normal limits.
In particular and surprisingly, it has now been found that the process according to the invention may preferably be carried out in such a manner that the desired racemic amino acids can be prepared without the aid of further substances and can then be obtained from the aqueous reaction solution with purities of  greater than 95% with, at the same time, a very high degree of selectivity.
In a preferred embodiment of the process of the invention, the hydrolysis is carried out at temperatures from 110xc2x0 C. to 220xc2x0 C., preferably from 140xc2x0 C. to 200xc2x0 C., particularly preferably from 140xc2x0 C. to 180xc2x0 C.
Racemic amino acids prepared in that manner are of high purity and can be used for many applications without further purification.
Suitable acylamino acids are compounds of the general formula R1xe2x80x94CH(NHxe2x80x94COxe2x80x94R2)COOH wherein R1 is hydrogen, a linear, branched or cyclic alkyl that has from 1 to 7 carbon atoms and that may contain substituents such as a hydroxyl, alkyloxy or alkylthio group. R2 represents a hydrogen atom or an alkyl having from 1 to 3 carbon atoms.
Examples of linear, branched or cyclic alkyls are methyl, ethyl, propyl, isopropyl, 1-methylpropyl, 2-methylpropyl, butyl and cyclohexyl, with alkyls having from 1 to 5 carbons being preferred. Such alkyls may be substituted by from 1 to 3 amino, hydroxyl, halogen, alkyloxy, alkylthio, urea or carboxy groups.
According to the invention, the acylamino acid to be hydrolyzed is heated together with water in a pressure-resistant vessel. The pressure that builds up corresponds to the vapor pressure of the water at the corresponding temperature.
The hydrolysis may be carried out continuously or discontinuously.
The amount of water required is dependent on the nature of the acylamino acid and is generally in a range from 0.3 to 2.0 liters (l), preferably from 0.5 to 1.5 l/mol of acylamino acid. The reaction time required for complete conversion is affected by the amount of water, an increase in the amount of water shortens the reaction time. It is also advantageous during the reaction to remove from the equilibrium by distillation the carboxylic acid that forms, or a portion thereof. However, it is also possible to separate off the carboxylic acid once the reaction is complete.
When the reaction is complete, the reaction solution contains, in addition to small residual amounts of the acylamino acid that was used, the amino acid that has formed, the carboxylic acid analogous to the acyl group used, and traces of condensation products. Isolation of the amino acid from the reaction solution may be effected by various methods known per se, the method depending on the solubility of the amino acid and the properties of the carboxylic acid. Before or after separation of the amino acid, or of a portion thereof, in the form of a crystallisate, the carboxylic acid is largely separated off by distillation, azeotropic distillation or extraction. Ketones, such as methyl isobutyl ketone, or ethers, such as methyl tert-butyl ether, may be used as the extracting agent.
For separation of the amino acid in the form of a crystallisate, the solution, particularly in the case of readily soluble amino acids, may be concentrated. The filtrate obtained after separation of the solids is a saturated aqueous solution of the amino acid and may be used in the hydrolysis again together with fresh acylamino acid.
The Examples which follow are intended to illustrate the invention, without having a limiting effect.